Rapid progress has been made in understanding the molecular mode of action of the Myc oncoprotein. Although the precise molecular mechanism of c-Myc activity in normal cell growth, oncogenesis, differentiation and apoptosis is unknown, numerous studies have uncovered a series of molecular and cellular properties of the protein which have led to a more complete understanding of its role in these opposing cellular processes. Myc acts within a network of interacting basic (b) Helix-Loop-Helix (HLH) Leucine Zipper (LZ) proteins. Together, Myc, Max, Mad, Mxi1, and Sin3 comprise a transcription factor super family in which the central protein is Max. In fact, all the family members (except Sin3) must dimerize with Max through their respective HLH and LZ domains to cooperatively bind DNA in a sequence specific manner. In contrast to the transactivating property of Myc/Max heterodimers, Max heterodimerization with Mad or Mxi1, followed by the tethering of mSin3, the mammalian homologue of the yeast repressor Sin3, to the target genes results in suppression of transcription. Cotransfection of c-myc and mad into murine erythroleukemia (MEL) cells provides a cell culture system which exploits not only the ability of Myc to block differentiation but also the ability of Mad to reverse the block. We found that Mad influences cellular growth and differentiation by an active repression mechanism rather than by simply competing for Max and occupying Myc/Max binding sites. Furthermore, we also discovered that a truncated Mad protein is more inhibitory to cell survival than wild type. Recent advances in the elucidation of protein-protein interactions which regulate progression through the cell cycle appear to present new opportunities for therapeutic intervention for the treatment of cancer. Disruption of Myc/Max dimerization via genetic manipulation has been shown to impair the transcription activation and cell transformation activities of these regulatory proteins. Utilizing the ELISA assay and short synthetic peptides spanning the HLH and LZ regions of Myc and Max, we detected a marked inhibition in the formation of Myc/Max complexes.